During the past decade, it has been determined that intergranular stress corrosion cracking (IGSCC) of sensitized austenitic stainless steels has been responsible for the occurrence of pipe cracks in boiling water nuclear reactors (BWRs). Research on intergranular stress corrosion has shown that the aggressiveness of 280.degree. C. water to sensitized austenitic stainless steels depends on the content of dissolved oxygen and certain ionic species present in the coolant water passed through the reactor. Data that has been obtained from such studies have shown that intergranular stress crack corrosion can be prevented by reducing the dissolved oxygen content of the cooling water to less than about 20 parts per billion (ppb) and maintaining the conducitivity of the coolant water below about 0.3 s/cm. It is suggested that the dissolved oxygen content of pure water must be reduced to less than 20 parts per billion to prevent intergranular stress crack corrosion of austenitic stainless steel, and even less than 10 parts per billion when reactor coolant water is utilized.
The described findings have led EPRI to perform an in-plant study at Dresden-2 in which hydrogen injection into the feedwater was used to reduce the dissolved oxygen content in the BWR coolant water. The intended outcome was the mitigation of intergranular stress crack corrosion in the reactor coolant recirculation lines of BWRs. Reports on such research have been presented in "Mitigation of Stress Corrosion Cracking in an Operating BWR via H.sub.2 Injections" by M. E. Indig and J. E. Weber, in Paper Number 124 presented at the International Corrosion Forum Sponsored by the National Association of Corrosion Engineers, Apr. 18-22, 1983; and in EPRI report "Controlling Stress Corrosion Cracking in BWR Piping by Water Chemistry Modifications", R. L. Jones, A. Machiels, M. Naughton and J. T. A. Roberts, in Paper 167, presented at the NACE Symposium on Corrosion Effects, Events and Control in the Nuclear Power Industry, Apr. 3-5, 1984. The EPRI process involves the injection of a large quantity of hydrogen into the feedwater, to suppress the radiolysis process which occurs into the reactor core and contributes dissolved oxygen to the reactor coolant water. This technique is successfully utilized in pressurized water nuclear reactor coolant water for oxygen control but in the absence of boiling of the reactor coolant water. The Dresden-2 data shows that, for a BWR with the EPRI-GE process, 1.8 parts per million (ppm) dissolved hydrogen must be added to the feedwater to suppress the oxygen content of the reactor coolant water recirculation lines from 200 ppb to less than 20 ppb. The EPRI study at Dresden-2 has shown that reduction of recirculation line oxygen content to less than 20 ppb has been effective in stopping intergranular stress crack corrosion progression. EPRI is recommending this alternate water chemistry that is called "hydrogen water chemistry" to all BWR plants for mitigating intergranular stress crack corrosion. At the presen time, hydrogen water chemistry has been implemented only at Dresden-2. Other BWR plants will probably switch over to a similar process. Although intergranular stress crack corrosion is sufficiently suppressed by the hydrogen water chemistry process, it is far from optimum from an operating viewpoint.
The disadvantages with the present hydrogen water chemistry process for combating intergranular stress crack corrosion can be summarized as follows: (1) inefficient hydrogen usage--large hydrogen usage for this process above stoichiometric requirements; (2) the need to process waste hydrogen gases resulting from the large hydrogen additions; and (3) a four to six-fold increase in the radiation exposure due to increased N.sub.16 releases, relative to no hydrogen addition. These disadvantages of hydrogen water chemistry stem from the need to add 1.8 ppm of hydrogen to the feedwater in order to suppress the core radiolysis so that a less than 20 ppb dissolved oxygen concentration in the reactor coolant recirculation line is obtained.
In the hydrogen water chemistry process, it is required that 1.8 ppm of hydrogen be added to the main feedwater to reduce the oxygen content of the recirculation lines from 200 ppb to 20 ppb. This process, however, only yields a minor reduction in the main feedwater oxygen content from 30 to 20 ppb. It appears that injection of hydrogen into the main feedwater is thus inefficient in the usage of the added hydrogen. For example, the reaction for the decomposition of water by radiolysis and the reverse reaction for water formation is the same, namely: EQU 2H.sub.2 +O.sub.2 .revreaction.2H.sub.2 O
where the forward reaction leads to the formation of water and the reverse reaction favors the dissolved gases. In a BWR core, the predominant reaction favors the left side of the equation unless an excess of hydrogen or oxygen is present. Computer modeling of radiolysis kinetics can be utilized to determine the minimum core hydrogen concentration which must be maintained to suppress oxygen production. It, however, may not be desirable nor practical to try to achieve this hydrogen concentration in the core. It is known that when hydrogen water chemistry was initiated at Dresden-2, the radiation levels at the plant increased by a factor of 4 to 6 above the pre-addition baseline. This increase in activity was attributed to an increase of N.sub.16 in the steam. The N.sub.16 is said to be formed in a reactor core by the nuclear reaction: EQU Oxygen.sub.16 +neutron.fwdarw.Nitrogen.sub.16 +proton.
Under normal water chemistry conditions, the N.sub.16 reacts with dissolved oxygen to form nitrate (NO.sup.-3) which is solube in the reactor coolant water. Under hydrogen water chemistry conditions, there is an insufficient amount of oxygen present to convert the N.sub.16 to nitrate. The N.sub.16 therefore ends up as a more volatile species such as ammonia and is removed from the water by steam. The decay of the N.sub.16 gives off high energy gamma thereby increasing the radiation exposure levels at a plant.
This explains one of the drawbacks of the hydrogen water chemistry process. In addition, the other drawbacks are related. The added 1.8 ppm hydrogen in the feedwater has a great difficulty in remaining in the reactor core due to boiling which occurs there. The stripping process from the boiling thus requires excessive hydrogen addition to maintain the minimum radiolysis suppression concentration of hydrogen in the reactor core. This also puts a tremendous burden on the off-gas recombiners, which now must handle the added discharge hydrogen, without the initially available discharge oxygen. This has necessitated injection of oxygen into the recombiner and the total operating cost for hydrogen water chemistry due to these problems has been estimated to exceed 500,000 dollars per year. Because of the complexity of the process in which oxygen is removed in the reactor core by adding hydrogen to the feedwater, and thus to the core, and then the off-gas hydrogen is removed by injecting oxygen into the recombiners, availability for the hydrogen water chemistry system has at times been less than 60 percent. Off-gas fires have also been frequent at Dresden-2 during the initial to 6 month test.
The intergranular stress crack corrosion has only been observed in the reactor coolant recirculation lines in which about 200 ppb or more of dissolved oxygen was detected. No such cracking was observed in the remainder of the feedwater piping where about 30 ppb dissolved oxygen was typically detected. Such an observation suggests that the optimum solution for the BWR intergranular stress crack corrosion problem would involve deoxygenation of the recirculation line without significantly altering the normal operating dynamics of the BWR water chemistry.
An object of the present invention is to effect the major oxygen removal of the reactor coolant water recirculation lines outside the core, to avoid the above identified disadvantages associated with conventional hydrogen water chemistry.